Pneumatically-operated braking system for vehicles

ABSTRACT

A supply dump valve comprises a piston ( 1 ) of substantial differential outline adapted to work in a stepped bore ( 2 ) in a housing ( 3 ). Actuation of a park brake causes the piston ( 1 ) to be exercised in response to operation of the park brake.

This invention relates to pneumatically-operated braking systems forvehicles and in particular to supply dump valves for systems of the kindfor use in tractor-trailer combinations.

Known pneumatically-operated braking systems for tractor-trailercombinations embody a safety feature, typically called a “supply dumpvalve”, that is operative to limit the loss of air stored in an airreservoir of the vehicle should a pneumatic control line between thetractor and trailer become ruptured or otherwise damaged. By restrictingthe loss of supply air the operation of the trailer emergency relayvalve is brought about. The supply dump valve itself can usually beintegrated into the trailer control valve which is fitted to thetractor. A trailer control valve for controlling operation of the brakeson the trailer under the control of an electronic braking system (EBS)is usually connected to the trailer through flexible couplings otherwiseknown as “Suzies”. The supply dump valve acts substantially to bringabout the operation of the trailer emergency relay valve and hence theapplication of the trailer brakes.

Conventionally the supply dump valve comprises a differential pressureactuated valve which only moves when a pressure differential acts acrossit. Known valves are fitted to both conventionally actuated pneumaticbraking systems as well as electronically controlled braking systems. Asthe supply dump valve only ever operates when a failure in the trailercontrol line occurs, the supply dump valve is classified as a redundantfeature. This means that the integrity of the dump valve should bechecked by some means to ensure that if a fault within the valve isdetected, the system can inform the operator at a stage before the valveis required actually to function. Alternatively it can be arranged sothat the valve is actually partially operated during normal operation soas to “work” whereby to reduce the possibility of the valve elementseither sticking or seizing when the dump valve is required to performthe function for which it is intended.

In one known construction the supply dump valve is made of adifferential area supported by a spring so that on each and every brakeapplication a differential force is generated by the application ofbrake pressure across a differential piston. This causes the piston anda seal acting between the piston and a bore in which it works to beexercised. A problem with this arrangement is that typically the valvewould not move until substantial brake pressure, perhaps between two andthree bar, had been applied. Since most vehicle stops fall within arange of 0-0.2 g which require less than 3 bar, typically only 0.5 to 1bar, the valve does not move and thus is not exercised reliably. In factit is unlikely, in a vehicle not subjected to substantial brakingaction, such as is experienced in near continuous motorway driving, forsufficient force required to exercise the supply dump piston to begenerated, except on only a few occasions.

According to one aspect the invention provides a supply dump valve for apneumatically-operated braking system for vehicles comprising a pistonof differential outline adapted to work in a stepped bore in a housing,characterised in that a supply pressure acting upon differential areascauses the piston to move in a direction to take up a small built inclearance, thereby earthing out the piston against a spring, and inwhich operation of a park brake causes a pressure rise in a deliveryline to said valve in turn to cause the piston to move in an oppositedirection until it engages with a stop so that actuation of the parkbrake causes the piston to be exercised.

This ensures that the piston is exercised at least once on each andevery journey.

Preferably the piston is of substantial diameter.

After release of the park brake the piston is seated upon the spring andupon application of the park brake the pressure in the delivery linedrives the supply piston into engagement with the stop.

Should a leak in the trailer control line occur, then the pressure inthe valve control chamber will drop causing the piston to move in adirection to throttle the line between the reservoir and the valve.

One embodiment of our invention is illustrated in the accompanyingdrawings which is a cross section through a supply dump valve for apneumatically-operated braking system for a vehicle of thetractor-trailer combination type.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawing thataccompanies the detailed description can be briefly described as ageneral sectional view of supply dump valve according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The supply dump valve comprises a piston 1 of substantial differentialoutline working in the stepped bore 2 in a housing 3. The differentialpiston 1 comprises a relatively short portion 4 of greater diameterworking in a complementary portion 5 of the bore 2, and an elongate stemportion 6 which defines a portion of smaller diameter and which works ina complementary portion 7 of the bore separated from the portion 2 by abore portion 8 of intermediate diameter. The stem defining the portion 6also projects above the portion 4 to define a guide 9 which works in abore 10 of a plug 11 for sealingly closing that end of the housing 3.The bore 10 is connected typically to the EBS inlet solenoid valvethrough radial passages 12 in the plug and passages 13 in the housingand those passages in turn, are connected to a reservoir port 14 in thehousing through an axial passage 15 which extends longitudinally betweenopposite ends of the piston 1.

A compression spring 16 acts between a slidable collar 17 and theadjacent face of the piston portion 4 which is of greater diameter. Thecollar 17 is adapted to co-operate simultaneously or alternatively witha shoulder 18 at the step in diameter between the bore portion 2 ofgreater diameter and the intermediate bore portion 8, and a circlip orsimilar member 19 carried by the piston portion 6.

Pneumatic control pressure acts on the face of the piston portion 4 ofgreater diameter through a passage 20 disposed between the plug 11 andthat face of the piston. Delivery pressure acts on the underside of theportion 4 of greater diameter through a passage 21. The supply of fluidto the valve through a passage 22 is adapted to be restricted or closedas the piston 1 moves relatively away from the end plug 11 and isreceived within a counterbore portion 23 at the adjacent end of the bore2.

When the supply dump valve is in the rest position illustrated in thedrawings the piston 1 is held in a neutral position with the force inthe spring 16 taken out on the circlip 19. Air from the reservoir port14 passes directly to the port 22 and also to the top of the piston 1.This pneumatic pressure subjects the piston 1 to a net force urging itrelatively away from the end plug 11 due to equal pressure acting overthe difference in areas between the portions 9 and 6 of the piston 1.The piston 1 moves downwardly until the collar 17 co-operates with theshoulder 18 and the force of the spring 16 resists further movement.Movement of the piston 1 in this direction takes place through a smallbuilt in clearance of substantially 2 mm.

Under normal braking the control pressure applied to the passage 20 andthe delivery air pressure are substantially equal so that the positionof the piston 1 does not change further.

Should a coupling or pipe leading to the trailer fail, then the deliverypressure acting on the underside of the portion 4 will fall causing thepiston 1 to move relatively away from the end plug 11 until it entersthe counterbore 23. This closes off the air supply to the valve andprevents air loss.

When a parking brake is applied there is a pressure rise in the deliveryline acting through the passage 21 but the pressure in the passage 20does not rise. This subjects the piston to a force in a direction tomove it relatively towards the end plug 11 until it contacts the endplug 11 which acts a stop.

The movement of the piston 1 towards the end plug 11 occurs during eachand every parking brake application. This ensures that the piston 1 doesnot stick in its bore and thus reduces the likelihood of it failing tomove during a failed pipe connection condition.

The foregoing description is exemplary rather than defined by thelimitations within. Many modifications and variations of the presentinvention are possible in light of the above teachings. The preferredembodiments of this invention have been disclosed, however, one ofordinary skill in the art would recognize that certain modificationswould come within the scope of this invention. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described. For thatreason the following claims should be studied to determine the truescope and content of this invention.

What is claimed is:
 1. A supply dump valve for a pneumatically-operatedbraking system for vehicles comprising a piston of differential outlineadapted to work in a stepped bore of a housing such that a supplypressure acting upon the differential areas causes the piston to move ina first direction to take up a small built clearance, thereby earthingout the piston against a spring to restrict a fluid supply passage froma reservoir port to an emergency valve in the event of an air failure,and in which actuation of a park brake causes a pressure rise in adelivery line to cause the piston to move in a second direction untilengaged with a stop so that each actuation of the park brake causes thepiston to be exercised.
 2. A dump valve according to claim 1, in whichthe piston includes a greater diameter piston portion, a smallerdiameter elongate stem portion and a guide portion projecting fromopposite faces of the greater diameter piston portion, pneumatic controlpressure acting on the upper face of the greater diameter piston portionleading to the guide, and delivery pressure acting on the underside ofthe greater diameter piston portion leading to the portion, and thespring acts between the underside of the greater diameter piston portionand a slidable collar, the slidable collar adapted to cooperatesimultaneously or alternatively with a shoulder in the housing at a stepin diameter, and a member carried by the smaller diameter elongate stemportion taking out the spring force in a rest position, and theresulting increase in delivery pressure caused upon applying the parkbrake urging the piston relatively towards the end of the bore ofgreater diameter.
 3. A dump valve according to claim 2, in whichreduction in the delivery pressure acting on the underside of thegreater diameter piston portion causes the piston to move relativelyaway from the bore of greater diameter until the smaller diameterelongate stem piston portion enters a chamber to close off the airsupply valve and prevent air loss.
 4. A supply dump valve for apneumatically-operated braking system comprising: a housing having astepped bore receiving a control pressure and a delivery pressure; astop member fitted to an end of said bore; and a piston movably fittedwithin said stepped bore, said piston having a piston portion, a guideportion, and a stem portion movable to restrict a fluid supply passagefrom a reservoir port to an emergency valve in the event of an airfailure, said piston portion having a diameter greater than said guideportion and said stem portion, the control pressure acting between afirst side of said piston portion and said stop member, the deliverypressure acting upon an opposite side of said piston portion, activationof a park brake increasing the delivery pressure relative to the controlpressure causing movement of said guide portion toward said stop memberso that each actuation of the park brake causes the piston to beexercised.
 5. The valve according to claim 4, wherein the deliverypressure is increased during actuation of a park brake.
 6. The valveaccording to claim 4, further comprising a biasing member to bias saidpiston to a neutral position.
 7. The valve according to claim 6, whereinsaid piston substantially maintains said neutral position during normalbrake application.
 8. The valve according to claim 4, wherein reductionof the delivery pressure causes the piston to move relatively away fromsaid stop member such that an end of said stem portion enters a chamberto throttle a passage and prevent fluid loss from saidpneumatically-operated braking system.
 9. The valve according to claim6, wherein said biasing member engages said piston portion and saidstepped bore.
 10. The valve according to claim 6, wherein said biasingmember engages said piston portion and a slidable collar which engagessaid stepped bore, said slidable collar slidable upon said stem portion.11. A method of controlling a valve for a pneumatically-operated brakingsystem comprising the steps of: (1) supplying a control pressure to apiston movably fitted within a stepped bore, said piston having a pistonportion and a stem portion, said piston portion having a diametergreater than said stem portion, the control pressure acting between afirst side of said piston portion and a stop member; (2) supplying adelivery pressure to an opposite side of said piston portion; (3)actuating a park brake to increase the delivery pressure relative to thecontrol pressure causing movement of said piston; and (4) biasing saidpiston to a neutral position such that said piston substantiallymaintains said neutral position during normal brake application.
 12. Amethod as recited in claim 9, wherein reducing the delivery pressurecauses the piston to move relatively away from said stop member suchthat an end of said stem portion blocks a passage to prevent fluid lossfrom said pneumatically-operated braking system.